Field of the Invention
This invention relates to a deep well electrical submersible pump and in particular to a pump for pumping fluids from a relatively small diameter wellbore under high load conditions.
When pressures in an oil reservoir have fallen to the point where a well will not produce at its most economical rate by natural energy, some method of artificial lift is employed. One of the lifting methods employed in such situations is that of a submersible electrical pump which is an especially built centrifugal pump, the shaft of which is directly connected to an electric motor. The entire unit is sized so that it may be lowered into the well on a pipe string commonly called tubing, to the desired operating depth. In operation, the motor causes the pump to rotate so that impellers in the pump apply centrifugal forces to the fluids entering the pump intake. The pump is installed on the production tubing below the fluid level in the wellbore. Since both the pump and the pump motor are submerged in the well fluid, electric current is supplied through a special heavy duty armored cable. The total pressure developed by such a pump forces fluid up the tubing string to the surface. The capacity of this type of pump can range from 200 to 26,000 barrels a day depending upon the depth from which the fluid is lifted and the size of the wellbore casing which determines the maximum diameter of the pump.
The electric submersible pump (ESP) is perhaps the most versatile of the major oil production artificial lift methods. ESPs are used to produce a variety of fluids and the gas, chemicals, and contaminants commonly found in these fluids. Currently ESPs are operated economically in virtually every known oil field environment. Relatively high gas fluid ratios can be handled using tapered designed pumps and/or a special gas separator pump intake. An ESP can be operated in a deviated or directionally drilled well. Although the recommended operating position is in a straight section, the ESP can operate in a horizontal position. ESPs have efficiently lifted fluids in wells deeper than 12,000 feet. The pumps can be operated in casings as small as 4.5 inches OD. Many studies indicate that ESPs are the most efficient lift method and the most economical on a cost per lifted barrel basis. The ESP historically has been applied in lifting water or low oil cut wells that perform similar to water wells. These pumps are typically constructed with impellers being mounted either fixed or floating on a vertical shaft, which when rotated, centrifugally force fluids outwardly and upwardly through a multiplicity of impeller diffuser stages to sequentially lift fluid to the surface. In effect, the stages of the pump sequentially pressurize the fluid so that the aggregate pressure increase can overcome the hydrostatic head within the fluid column above the pump and thus eventually move the fluids to the surface. These pumps are designed to minimize the effect of hydrostatic pressures in the wellbore on the pump parts. This is typically done by the utilization of balancing hubs or drums to minimize forces within the pump to prevent any unnecessarily high forces from being imparted to the parts thereof which would in turn impose high frictional forces on the moving parts therein to generate excessive wear of the parts. The hydrostatic forces which are encountered at the pump level in such a well typically are a result of the height of the fluid column in the tubing string above the pump which is acting down upon the pump parts. In a large diameter wellbore it is possible to use a pump of sufficient diameter to employ a large thrust bearing. Such a large thrust bearing is capable of absorbing greater loads which may be imposed upon the pump. However, in a small diameter bore hole, the thrust-bearing size is compromised to the extent that it may not be sufficient to withstand the downward forces exerted upon the pump shaft in deep well applications. In this case such forces acting on the pump parts may generate wear to the extent that such a pump system is impractical.
One design which has been used to overcome this problem of excess force on the pump shaft, is that of a bottom floater pump. In such a pump, impellers on the upper end of the pump are fixed to the pump shaft. Therefore, a portion of the load on the pump shaft due to hydrostatic pressure acting on the cross-sectional area of the shaft, is transmitted to the impellers fixed on the shaft. The impellers in turn have thrust washers which engage mating surfaces on the diffusers which in turn are connected to the pump housing so that the load of the pump shaft is partially absorbed eventually by the pump housing, which is carried by the tubing string thus relieving the load on the thrust bearing. The bottom impellers in such a pump are permitted to float on the pump shaft so that thrust loads on the impellers are not transmitted to the shaft and vice versa. This bottom floater impeller design has been frequently employed in small diameter pumps, such as being run into deep wells, when it is not desired to impart heavy loads onto the thrust bearings which are limited in size by the small housing diameter available. However, when the operating depth of the well is such that the hydrostatic forces operating on the pump shaft become excessive, the small thrust bearing which is dictated by the small diameter pump housing is not able to withstand the thrust loads even though a portion of the shaft load is transferred to the pump housing by way of the fixed impellers on the shaft. Additionally, in the bottom floater system just described, as the bore hole depth increases, the down loading of the shaft which is transferred to the pump parts causes wear on the pump parts to the extent that the system is no longer practical.
These problems associated with wear to bearing surfaces created by such deep well pumps has in the past also been treated to a certain extent by the use of balancing hubs or the like which attempt to provide pressure balance on pump components so that friction surfaces which are caused to engage one another are placed as near as possible under balanced force conditions, to thereby minimize the frictional forces acting on such engaging surfaces. An example of this is shown in U.S. Pat. No. 2,809,590 to Brown which shows an electric motor driven pump wherein discharge pressures are rerouted back into the pump system to act upwardly on pump surfaces against which such discharge pressure are being imposed in order to generate a pressure balance and thus minimize wear forces acting on the relatively moving parts. This is done by providing a pressure balancing disk which is mounted on the pump shaft and internal pressure balancing passages are used to bring a fluid pressure differential to the balancing disk.
U.S. Pat. No. 4,793,777 to Havenstein shows a centrifugal pump including an axillary impeller arranged additionally to the pump impeller proper to provide for pressure reduction and a throttling device to bring about an equalization of thrust forces acting on the impeller.
Vitu U.S. Pat. No. 1,867,290 also describes a centrifugal pump wherein openings are provided in the impeller to permit the passage of at least part of the volume of the liquid being handled by the pump to the back side of the impeller in order to balance pressures on the two sides of the impellers.
Peterson U.S. Pat. No. 1,609,306 also shows a balancing disk for adjusting forces of a centrifugal well pump.
In each of the above systems an attempt is made to balance pressures acting on a surface by transmitting through some means the higher discharge pressures to lower pressure surfaces in the apparatus to thereby balance forces acting on the various parts. Some of these systems are rather simple and yet others are very complex, but in any event, they are not sufficient or practical to deal with the extremely high forces that are encountered in deep well operations contemplated by the present invention.
It is, therefore, an object of the present invention, to provide a new and improved pump system which will obviate the load problems occurring in deep wells having small diameter pumps by providing a net upthrust lifting force on selected impellers which upthrust is transmitted to the pump shaft to partially offset shaft forces acting down on the thrust bearing.